[cpp] make sure StringMap's h field is kept if we use StringMap

(cherry picked from commit 7ff9b212782c1224c5c6a93d0a8a784badb836c0)

std/cpp/_std/haxe/ds/StringMap.hx

@@ -46,6 +46,7 @@ package haxe.ds;
   inline Void set(Dynamic &key, const VALUE &value) { set( (String)key, value ); return null(); }
 ")
 @:coreApi class StringMap<T> implements haxe.Constraints.IMap<String,T> {
+	@:ifFeature("haxe.ds.StringMap.*")
 	private var h : Dynamic;
 
 	public function new() : Void { }